As is well understood, heavier-than-air vehicles, other than gliders, require power to generate the thrust that allows them to move through the air in order to create lift and stay aloft. Most of these air vehicles use reciprocating or jet engines that burn fossil fuels as the primary power source. Although, these engine powered air vehicles provide high performance and maneuverability, they are limited in range and time aloft by the amount of fuel that they can carry. Gliders, on the other hand, may stay aloft for indefinite periods of time, but are limited in utility by the unpredictable nature of the prevailing winds.
With the threat of global warming and increasing gasoline prices, the interest in solar powered aircraft using photovoltaic solar cells has increased. Photovoltaic technology is well established, having been used as the primary power source on satellites for many years. An advantage of a solar powered aircraft over an engine powered aircraft is that it can stay aloft for indefinite periods that may extend into years of time similar to the operation of a satellite. Further, the solar powered aircraft can perform many of the present functions of a satellite without the cost of an expensive launch vehicle and without eventually creating orbital waste. The solar powered aircraft can return to earth for maintenance and be re-configured for a variety of missions. Typical applications include surveillance and tracking, homeland security, communications, oceanography, and meteorology. Many of these applications require operation at high altitudes greater than 60,000 feet where air breathing engine are less efficient.
In order to operate during periods of darkness, solar powered aircraft must employ a means to store the energy created by the solar cells. Batteries and fuel cells are the devices that have been employed, to date, on solar powered aircraft to store energy. Key drivers for energy storage devices used in this application are long cycle life, high energy per unit weight, and high efficiency. Primary fuel cells must store hydrogen and oxygen for fuel and therefore are limited in life in a similar fashion as fuel burning engines. Regenerative fuel cells have not demonstrated long cycle life at this time. Recent advances in Lithium-Ion, Lithium-Sulfur, and Lithium-polymer batteries has increased energy per unit weight, but the available energy of these technologies over the cycle life and the harsh environment required for long life high altitude air vehicles is still questionable. As a consequence, batteries must be thermally managed, particularly at high altitudes where temperatures can reach −76 degrees Fahrenheit, thereby consuming energy that could be used by payloads. To date, solar powered flight for indefinite periods of time has not been achieved with batteries or fuel cells.
A more appropriate energy storage device for use with long life solar powered airplanes is a high speed flywheel with a composite rotor and magnetic bearings. High speed flywheels provide superior energy per unit weight than batteries in applications where long cycle life is required. Composite rotors have demonstrated over 112,000 cycles in laboratory tests. Furthermore, flywheels provide a more efficient energy to thrust ratio for this application than batteries since batteries store energy chemically. Converting electrical energy to chemical energy, then back to electrical energy and then to rotational energy is inherently less efficient than converting the electrical energy directly to rotational energy in the flywheels and then using torque conversion to apply the energy directly to rotating propellers. Furthermore, flywheels produce moments and gyroscopic couples that can be used to steer the aircraft and eliminate or reduce the need for additional steering devices such as flaps, fuselage and a tail section that create additional drag on the vehicle. To date, flywheel energy storage has not been used in solar powered air vehicles.
The following patents disclose solar powered aircraft that store the solar power in batteries or similar chemical based storage methods. U.S. Pat. No. 3,089,670 issued May 14, 1963 to E. G. Johnson, U.S. Pat. No. 4,415,133 issued Nov. 15, 1983 issued to William H. Phillips, U.S. Pat. No. 4,697,761 issued Oct. 6, 1987 to David E. Long, U.S. Pat. No. 5,518,205 issued May 21, 1996 to Wurst et al U.S. Pat. No. 5,810,284 issued Sep. 22, 1998 to Hibbs et al. and U.S. Pat. No. 7,198,225 issued Apr. 3, 2007 to Lisoski et al. A major problem with storing the power in batteries is the conversion of the energy solar power into battery power and back into solar power. An additional drawback is operation of batteries at high altitude where the temperature is colder. Energy from the batteries must also be used to keep the batteries warm for optimal operation.
In U.S. Pat. No. 7,198,225, Lisoski, et al., achieved high altitude solar powered flight with fuel cell energy storage for several hours. In U.S. Pat. No. 5,921,505, Spector derives a method for reducing disturbances in energy storage flywheels in solar powered craft such that the flywheels do not interfere with an on-board dynamic control system. The Spector patent does not address the need for efficiently flying a solar powered air vehicle using flywheels as the only method for controlling the vehicle.
Therefore, a need remains to develop a solar powered air vehicle that can fly for extended periods of time which overcomes the disadvantages discussed above and previously experienced. Such a vehicle must be extremely lightweight with a high lift, low drag platform. The power train for this vehicle must provide highly efficient conversion of energy to thrust.